Never had to build one yet, but here are some things I'd consider:

How would it attach to the rest of the dash? Is there a piece there already with things like clips? Then I'd consider using that piece and do any cutting and buildup neccessary with it as the base.

For building up I'd consider fibreglass and resin (larger fairly smooth areas) or putty with glass fibres in it (backside reinforcements, packing an existing mold, smaller areas). If you need buildup with no real strength I'd look at the stuff the guy that built the Y2K Bug computer "case" used, some form of 2 part putty [Milliput seems to be the name] that hardens after being kneaded together.

It might also be possible to do it all in wood, solid or something like MDF. Stack and glue for the required thickness, cut/route out the outer shape and the DIN size opening, then cut, grind, rasp, file and sandpaper to the wanted shape.

Whatever way you go, you'll want an easy to sand putty to finish off the surface with (before painting or covering).

/Michael